The invention relates to a zeolite catalyst and a method for the reduction of oxides of nitrogen present in waste gases containing oxygen, using this catalyst The reduction takes place in the presence of ammonia, in which method, the waste gases containing nitrogen oxides are mixed with ammonia gas and passed at an elevated temperature over a zeolite catalyst containing a subgroup metal.
Nitrogen oxides, which arise in combustion processes, are among the main causes of acid rain and the environmental damage associated therewith. Therefore, their emission should be prevented by removing the nitrogen oxides from the waste gases before they are released into the environment.
Sources for the emission of nitrogen oxide into the environment are vehicular traffic; combustion plants, especially powerplants with furnaces; stationary internal combustion engines; and industrial plants. Vehicular traffic is a major contributor to the pollution problem.
A reduction of the nitrogen-oxide concentration in waste gas can be achieved in powerplants with boiler firing by using very pure fuels or by optimizing the combustion systems; however, these firing measures are subject to technical and economic limits. Because of these limitations, it has not been possible in the past to produce waste gas which is largely free of nitrogen oxides.
Waste gases from furnaces or from internal-combustion engines operated in an excessively stoichiometric manner usually contain a rather large amount of excess oxygen, in addition to the nitrogen oxides.
In order to assure an optimum utilization when employing reducing agents, selective, catalytic reduction methods are primarily considered for economic reasons for the removal of nitrogen oxides from waste gases.
It is already known that nitrogen oxides can be selectively reduced by treatment with a reducing gas, such as ammonia, in the presence of a catalyst. The ammonia gas reacts readily with the oxides of the nitrogen, but it reacts only to a slight extent with the oxygen.
In the catalytic reduction of nitrogen oxides, e.g. with NH.sub.3, so-called full catalysts are used, e.g. in the form of bulk material or extrudate bodies, which exhibit numerous parallel conduits running in the direction of flow of the gas (monoliths or honeycombs). The term "full catalyst" denotes a catalytic body which comprises an active mass of catalytic material throughout. The active mass may also contain additives of carrier material in addition to the actual catalysts.
As a rule, subgroup metal oxides, e.g. oxides of vanadium, tungsten, molybdenum or iron, are used individually or in combination as catalytically active components (see DE-PS 24 58 888). Carrier materials with catalytically enhancing action can be TiO.sub.2, BaSO.sub.4 or Al.sub.2 O.sub.3 (DE-PS 24 58 888 and DE-PS 28 42 147). In certain instances, such catalysts may also be compounded with noble metals such as platinum or palladium (U.S. Pat. No. 4,018,706).
Full catalysts in monolithic or honeycomb form are used because of the considerable abrasion and the low danger of clogging due to dust particles. This is problem in particular in the waste-gas range of furnaces, in which range the waste gas is still charged with dust. In instances in which the waste gas accumulates essentially dust-free, bulk catalysts or monolithic carrier catalysts may also be used.
Carrier catalysts with monolithic or honeycomb structure are better suited than bulk catalysts for the removal of nitrogen from waste gases with a low dust content. This is because the monolithic or honeycomb structure has a lower flow resistance. These catalysts are preferably used after a dust filter or behind a flue-gas desulfurization system.
These carrier catalysts include a catalytically inert structural strengthener with a coating of catalytically active material deposited on it. The catalytically active material can be mixed with a carrier material which acts to increase the surface area or as a so-called interspersant
The structural strengthener usually used for this application are ceramic or metallic, chemically inert carriers with a honeycomb structure consisting e.g. of cordierite, mullite, .alpha.-aluminum oxide, zirconia, zirconium mullite, barium titanate, porcelain, thorium oxide, steatite, boron carbide, silicon carbide, silicon nitride or fine steel.
All of the known catalysts have a number of disadvantages. The oxides of vanadium and molybdenum can be carried out of catalyst formulations containing vanadium and molybdenum by sublimation or by abrasion caused by the flue dust. These oxides contribute to the undesired contamination of the separated flue dust and the gypsum, produced in the desulfurization of flue gas with heavy metals.
Catalysts containing iron oxide with a high iron content can be irreversibly deactivated in sulfurous waste gases by means of sulfate formation.
The reactivation of used catalysts containing heavy metals, especially those containing vanadium and/or tungsten, for the purpose of removal of the heavy metal has not yet been evolved in such a fashion that a fully-developed method which is technically and economically satisfactory is available. Therefore, they can only be disposed of at a depository at the present time. Moreover, strict work area protection measures must be observed even during the production of such catalysts.
It has already been suggested in Chem. Letters 1975, (7), pp. 781-4 that zeolites of the Y-type containing copper and/or iron be used for the reduction of waste gases containing NO.sub.x.
However, this zeolite type has proven to be very sensitive to acidic components in waste gases which are produced, for example, during the combustion of fossil fuels such as brown coal, hard coal or petroleum (SO.sub.2, SO.sub.3, HCl, HF or NO.sub.2). An additional problem is the fact that water vapor is present in the mentioned waste gases in concentrations of approximately 8 to over 20% by volume.
It has also turned out that the specified zeolite structure is destroyed by the action of acid both at temperatures above the dew point and also if the dew point is dropped below at times, little by little by the process of dealuminization. This results in service lives for catalysts of this type which are too short to be of practical use.
DE-OS 33 28 653 indicates the use of a ceramic molecular sieve for the same application whose channel diameter should range from below the critical molecular diameter of ammonia to above the critical molecular diameter of nitrogen The channel diameter should preferably be between 2.5 and 4.0 angstrom units. A sufficient definition of the molecular sieve in question is lacking in this publication; according to the data, it cannot be mordenite.
However, one of its features should reside in the fact that its surface should be free of any catalytic coating. Quantified data regarding activity is lacking.
DE-OS 36 35 284 suggests the use of a zeolite of the ZSM-5 type in the H form charged with subgroup metals for the same application This catalyst is stable against acids, but its ion-exchange capacity is very small. In addition, the diameters of the zeolite pores are so small that it can be assumed that the indicated subgroup metals are not present in a form which is bound to the zeolites, but rather, in the form of an oxide precipitated onto the zeolite grain. The zeolite material thus probably functions only as carrier material for the subgroup metal oxides.